Experiment 6: Dehydration Of 2

Experiment 6: Dehydration of
Reading: Mohrig, Hammond & Schatz
Carey & Guiliano
Ch. 19 pgs 256-276
Ch. 5 pgs 184-207
Ch. 6 pgs 250-253
Dehydration of 2-Methylcyclohexanol
• This week's reaction:
- dehydration of a 2° alcohol to give a mixture of alkene isomers
- H3PO4 is a catalyst (facilitates reaction, but is not consumed)
- water is lost as reaction proceeds
• Which product will be major?
• Why? Zaitsef's rule - most highly substituted (most stable) product is preferred
• How does this reaction occur (what mechanism)?
Reaction Mechanism
• E1 elimination:
• Note: reaction is reversible!
- drive to completion by removing product from reaction mixture
e.g. distill - products are volatile! (boiling points?)
Types of Chromatography
1. Thin Layer Chromatography (TLC)
• stationary phase: spread over glass or plastic sheet
• mobile phase: liquid; drawn up plate by capillary action
2. Column Chromatography
• stationary phase: contained in a column
• mobile phase: liquid; passes through column
(gravity or pressure)
 3. Gas Chromatography (GC)
• stationary phase: contained in a column
• mobile phase: gas; passes through column
Gas Chromatography
• Useful analytical technique
- follow reaction progress (requires a standard)
- use for product identification (requires a standard)
- determine product purity
- determine ratio of product or isomers (quantitative measure!)
• Extremely sensitive
- can separate very complex mixtures (hundreds of components)
- don't need much material (typically 10-6 - 10-15 g  µL)
• Often used in environmental & forensic labs
- identification of trace amounts of material
• Basic principles, same as other forms of chromatography, but:
- stationary phase: non-volatile polymer on glass or solid support
- mobile phase: inert gas - typically He
- column length: 10 - 30 feet
- temperature: 25-300°C
Gas Chromatography
• How does it work?
- long column packed w/stationary phase & placed in an oven
- inert gas (mobile phase) passed through column at a controlled flow
- liquid sample injected (µLs), vaporized as passes through column, &
detected as it emerges
• Compounds travel through column
at different rates depending on:
- volatility
- strength of interaction with
stationary phase
 lower boiling compounds tend
to come off first
• Longer GC column leads to better
Gas Chromatography
• Detector response plotted vs. time
• Retention time: time it takes a compound to emerge
length of time compound is retained in the column
Retention time =
distance from injection point to sample peak (cm)
chart speed (cm/min)
• RT will remain constant for a given set of conditions (column type & temp)
Gas Chromatography
• detector response is proportional to amount of compound passing
through it  can quantify amounts by comparing peak areas
Peak Area = (Peak Height) x (Peak Width at 1/2 Height)
0.4 cm
0.5 cm
4.1 cm
1.2 cm
Peak Area X = (1.2 cm) x (0.4 cm) = 0.48 cm2
Peak Area Y = (4.1 cm) x (0.5 cm) = 2.05 cm2
Ratio X : Y = 1 : 4.3
Sample GC Trace - cyclohexane:toluene (1:1)
retention time
peak areas
note baseline separation of peaks
Next Week
(October 18 - 22)
Experiment 6: Dehydration of an Alcohol/Analysis by GC
A. Synthesis of a mixture of alkene isomers
dehydration of 2-methylcyclohexanol
isolate products by distillation
B. Analysis
bromine test for unsaturation
GC Analysis: determine ratio of two double bond isomers
Column Chromatography Lab Report (exp 5)
Lab Reports are due at the beginning of your regular lab session
Experimental Details
1. Combine reactants in long neck roundbottom flask
carefully combine 2-methylcyclohexanol and 85% phosphoric acid
wear gloves! phosphoric acid can cause burns
2. Add a boiling chip & assemble a modified fractional distillation apparatus
(a long neck flask with stainless steel sponge in the neck)
will drive reaction forward by removing product alkenes as they form
water co-distills with your product
4. Before you begin
check to be sure all joints are sealed & clamped
check the thermometer position!!
5. Proceed with distillation
cool the collection vial in ice (products are volatile!)
do not allow temperature of distillate to exceed 96°C
CHANGE!! do not tent apparatus with foil
do not distill the flask to dryness!;
6. Stop distillation; Keep the collection vial COLD!!
connector with
support rod
roundbottom flask
packed with
stainless sponge
boiling chip
ice bath
sand bath
Experimental Details
7. Wash distillate with NaHCO3 (you are doing a mini extraction!)
add solid NaCl to product vial to saturate the aqueous layer
add 1 mL saturated NaHCO3
mix well by drawing layers in & out of a disposable pipet
check pH to be sure any acid has been neutralized
8. Draw off aqueous layer using a pipet
which layer is the aqueous layer???
the bottom layer!
keep everything just to be sure!
9. Dry organics over anhydrous Na2SO4
cap & keep vial cold while drying
10. Transfer the dried product to a clean, dry, preweighed (with cap) vial
include the cap when you pre-weigh the vial
11. Weigh product in capped vial
mass of product can be used to calculate the percent yield
12. Analyze the product
bromine test, TLC, IR, GC can be done in any order
Bromine Test for Unsaturation
• Qualitative Test - helps identify what functional groups are present
in this case we are looking for the presence of a C=C
can be detected by addition of bromine to a solution of an organic
positive test is decolorization of the solution (colored  colorless)
• How it Works: Halogenation of the Double Bond
Thin Layer Chromatography
• Will visualize spots using a chemical stain - KMnO4
dip plate, let dry on paper towel
do this in the hood ONLY!! KMnO4 is a strong oxididant!
• This is a destructive method of visualization
compounds undergo a chemical reaction on the TLC plate
will see light spots on a purple background
may take some time to develop
Writing the Lab Report: Exp #5 Column Chromatography
 Purpose
- technique experiment: what will you learn?
- what conclusions will you reach?
- a general discussion of theory/expected results is not a purpose!
 Results & Discussion
- Identify contents of each flask (flask #1 and flask #2)
which is ferrocene? which is acetylferrocene
clearly explain how you reached this conclusion
what data can you use?
TLC - correlate Rf to structure
color (what color should it be?)
discuss success of separation
are the fractions pure?
how efficient was the separation (e.g. discuss your % recovery)
be sure to justify your answers using the data/info available
Writing the Lab Report: Exp #5 Column Chromatography
 Conclusion
- a brief recap of your findings
- include a general statement about column chromatography
(what did you learn?)
- do not include a lot of theory
 Appendix A: Calculations
- Rf
- Percent Recovery
 Appendix B: Spectra
- Page of IR spectra (provided)